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Implement Spy decorator

The Decorator is a design pattern that allows you to modify the existing functionality of a function by wrapping it in another function.

In the JavaScript testing framework Jest, a "spy" is typically used to monitor the behavior of a function:

spyOn(object, methodName);

You are required to implement Spy Decorator, which counts how many times the given function was called and with which arguments.

Example

const obj = {
  counter: 1,
  add(num) {
    this.counter += num;
  },
};

const spy = spyOn(obj, "add");

obj.add(1); // 2
obj.add(2); // 4

console.log(obj.counter); // 4
console.log(spy.calls); // [ [1], [2] ]

Solution​

  • Let's create a spyOn function which takes two arguments:
    • obj - the object containing the method to be spied on
    • methodName - a string representing the name of a method
  • Let's initialize an array calls, which is used to store the arguments with which the spied method is called.
  • Create originMethod which will be the reference to the original method from the object. This is necessary to maintain the original functionality of the method while adding the spy capability.
  • Ensure that the specified methodName corresponds to a function in the object. If not, an error is thrown. This is a safeguard to prevent incorrect usage of the spyOn function.
  • The original method on the object is then overridden with a new function. This new function does two things:
    • It records the arguments passed to the method call in the calls array.
    • It calls the original method using apply to ensure that the context (this) and arguments are passed correctly.
  • Finally, the spyOn function returns an object containing the calls array. This allows the user of the function to inspect the arguments with which the spied method has been called.

Code​

function spyOn(obj, methodName) {
  const calls = [];

  const originMethod = obj[methodName];

  if (typeof originMethod !== "function") {
    throw new Error("not function");
  }

  obj[methodName] = function (...args) {
    calls.push(args);
    return originMethod.apply(this, args);
  };

  return {
    calls,
  };
}